I won't talk about what the QR code looks like, but talk about the concept, history, classification, structure and implementation.
1. The concept of QR code :
2. The development history of QR code:
3. One-dimensional code and two-dimensional code:
4. Classification of QR codes:
5. Two-dimensional code classification - linear stacked two-dimensional code:
6. Two-dimensional code classification - matrix two-dimensional code:
7. Two-dimensional code classification - postal code: 
8. The advantages of two-dimensional code:
The QR code can accommodate: 1850 uppercase letters/2710 numbers/1108 bytes/500+ Chinese characters
9. Disadvantages of QR code:
10. International standards for QR codes:
11. About QRCode:
12. Two-dimensional code symbol structure:
The domestic standard of QR code can refer to "GB/T 18284-2000, QR code national standard, QR code national standard", the document has been uploaded to the attachment, and those who are interested can download it.
Finally, thanks to the teachers of MOOC!